[flexiglass] View-model integration test.

Adds a view-model level integration test for Flexiglass. This is meant
to be a collection of all known "rules" related to how Flexiglass
automatically changes scenes based on device state like authentication
and wakefulness.

Fix: 294899205
Test: Built and ran, manually made sure that all scenarios described in
the test class work as intended.

Change-Id: I49577bc72c838b3afbb8d00931b2a388aeaf0a60